I'm having trouble figuring out your question. Perhaps you are ESL?
'course maybe I'm wrong about the options since I don't use them, but…
"opendns" and "local dns" have nothing to do with one another.
As far as the local cache size……
the fact that you ask this question implies that you shouldn't be asking it because you don't have the requsite background knowledge.
The answer is:
1) Adequate. Yeah, I know that's not a number. We picked the exact value by investigation. FWIW, my main router is now on 21 days uptime, my network has one box dedicated to an active torrent client, and in 21 days has never had a local cache overflow (i.e., no unexpired cache entries deleted).
Actually, the size is not very important as long as it is more than a few dozen.
2) "requsite background knowledge"? That you can examine the current configuration with ps and cat. You would have seen the answer immediately.
3) Google is your friend.
4) Gratuitous editorial comment: Unles you really really really know what you're doing, you should not mess with the config of the local dns server. It's complex, poorly documented, easy to mis-configure, and not really necessary—-the default config parameters are very good.